Pipeline steel welded heat affected zone microstructure and mechanical properties of expansion features
Hot ya steel pipe welding area of organizations can be divided into two categories: not easily quenched steels, alloy steels and hardened steels. Easily quenched steel changed after the formation of the fusion zone, coarse grain zone, recrystallization, incomplete crystallization area and aging embrittlement; hardened steel transformation after the formation of CUI acquisition, not entirely quenched and tempered.
Hot ya steel pipe welding of pipeline steel is difficult for hardened steel, welding heat-affected zone of the fusion zone and coarse grain area for material loss larger, easy to form a brittle, the degree of damage depends on the base metal alloy system, State of original microstructure of base metal prior to welding and welding parameters. Below the X65.X70.x60.x40.L360 level of pipeline steel, online energy is low among produce ferrite and Pearlite, and easy to produce martensite (m), upper Bainite (Bu) and granular Bainite (Bg); online at high energy, coarse grain area easy to produce ferrite and Pearlite, but is also easy to produce the eutectoid ferrite and widmanstatten structure. Generally, upper Bainite and proeutectoid ferrite and widmanstatten structure is a bad organization of embrittlement. For more than X70.L360 grade of acicular ferrite pipeline steel, coarse grain zone mainly to Bainite (plank granular Bainite and Bainite), massive proeutectoid ferrite and ferrite. On the plank or massive ferrite or ferrite Matrix block MA on the island. This coarse grain zone toughness of steel caused by the major factors are:
(1) the MA relative to the composition, size and shape. Ma more, is too thick or too long, and the uneven distribution of embrittlement phenomena such as severe.
(2) not easy welding in welding of pipeline steel, front behind weld fracture, so it must be welded in front behind the insulation, use high grade welding consumables.
(3) the effective grain size or base metal grain growth tendency. With the increase of energy, not only prior austenite grain size and secondary structure thicker, bigger. Lath ferrite reduction as well as the massive increase of ferrite became one of the major cause embrittlement of coarse grain area.